Adenoviruses are being used to study regulation of the sequential reactions in viral replication and the effects on their host cells. These viruses are particularly useful for this objective since they can establish productive or abortive infections, depending upon the host cell, leading to either cessation of cell division or transformation of the cells to unlimited growth. Conditionally lethal, temperature-sensitive (ts) mutants are proving to be of particular value for investigating these control mechanisms. Special use will be made of mutants unable to initiate DNA replication at the non-permissive temperature owing to a defective DNA-binding protein (e.g., H5ts125) for studying control of DNA replication, transcription, and transformation. New techniques will be employed to isolate additional mutants in early gene functions, and ts mutants of type 7 adenovirus will be isolated and characterized.